The practical division of biology, according to the extent of the organic kingdom, leads us to mark out four chief provinces of research: protistology (the science of the unicellulars), botany (the science of plants), zoology (the science of animals), and anthropology (the science of man). In each of these four fields we may then distinguish morphology (the science of forms) and physiology (the science of functions) as the two chief divisions of scientific work. The special methods and means of observation differ entirely in the two sections. In morphology the work of description and comparison is the most important as regards both outer form and inner structure. In physiology the exact methods of physics and chemistry are especially demanded--the observation of vital activities and the attempt to discover the physical laws that govern them. As a correct knowledge of human anatomy and physiology is indispensable for scientific medicine, and the work requires a particularly large apparatus, these two sciences have long been studied separately, and have been handed over to the medical facility in the division of the academic curriculum. The broad field of morphology may be divided into anatomy and biogeny; the one deals with the fully developed, and the other with the developing, organism. Anatomy, the study of the formed organism, studies both the external form and the inner structure. We may distinguish as its two branches the science of structures (tectology) and the science of fundamental forms (promorphology). Tectology investigates the features of the structure in the organic _individual_, and the composition of the body out of various parts (cells, tissues, and organs). Promorphology describes the real form of these individual parts and of the whole body, and endeavors to reduce them mathematically to certain fundamental forms (chapter viii.). Biogeny, or the science of the evolution of organisms, is also divided into two parts--the science of the individual (ontogeny) and of the stem or species (phylogeny); each follows its own peculiar methods and aims, but they are most intimately connected by the biogenetic law. Ontogeny deals with the development of the individual organism from the beginning of its existence to death; as embryology it observes the growth of the individual within the fœtal membranes; and as metamorphology (or the science of metamorphoses) it follows the subsequent changes in post-fœtal life (chapter xvi.). The task of phylogeny is to trace the evolution of the organic stem or species--that is to say, of the chief divisions in the animal and plant worlds, which we describe as classes, orders, etc.; in other words, it traces the genealogy of species. It relies on the facts of paleontology, and fills up the gaps in this by comparative anatomy and ontogeny.